Everycanceroriginates from a single cell. During expansion of the neoplastic cell population, individual cells acquire genetic and phenotypic differences from each other. Here, to investigate thenatureand extent of intra-tumour diversification, we characterized organoids derived from multiple single cells from threecolorectalcancers as well as from adjacent normal intestinal crypts.Colorectal cancercells showed extensive mutational diversification and carried several times more somatic mutations than normalcolorectalcells. Most mutations were acquired during the final dominant clonal expansion of thecancerand resulted from mutational processes that are absent from normalcolorectalcells. Intra-tumour diversification of DNA methylation and transcriptome states also occurred; these alterations were cell-autonomous, stable, and followed the phylogenetic tree of eachcancer. There were marked differences in responses to anticancer drugs between even closely related cells of the same tumour. The results indicate thatcolorectal cancercells experience substantial increases in somatic mutation rate compared to normalcolorectalcells, and that genetic diversification of eachcanceris accompanied by pervasive, stable and inherited differences in the biological states of individualcancercells.